This invention relates to a digital video tape recorder (VTR) having a track format in which digital video and audio signals are recorded in predefined areas on helical tracks, wherein digital video and audio signals are input in the form of a bit stream, and are recorded.
FIG. 46 shows the track pattern of an ordinary consumer digital VTR. In the figure, helical tracks are formed on a magnetic tape 200, each being divided into a video area 201 for recording digital video signals and an audio area 202 for recording digital audio signals.
There are two ways of recording video and audio signals on this consumer digital VTR. One is baseband recording wherein analog video and audio signals are input and high-efficiency coding is performed on them. The other is transparent recording wherein a digitally transmitted bit stream is recorded.
The latter, transparent recording is more suitable for recording ATV (advanced television) signals now being discussed in the U.S.A. This is because ATV signals are already digitally compressed, do not require a high efficiency coder or decoder, and can be recorded as they are, with no picture quality deterioration. The problem however is picture quality during special playback such as fast, still and slow playback. Specifically, almost no pictures are reproduced during fast playback if a bit stream is recorded on the helical tracks as they are.
One method of using a digital VTR to record these ATV signals is described in a "A Recording Method of ATV Data on a Consumer Digital VCR", a technical presentation given at the "International Workshop on HDTV '93 held from 26-28 October, 1993 at Ottawa, Canada. This technique will now be described as an example of the prior art.
According to one basic specification of a prototype consumer digital VTR, in the SD (Standard Definition) mode, one video frame is recorded in video areas of 10 tracks if the recording rate of the digital video signal is 25 Mbps and the field frequency is 60 Hz. If the data rate of an ATV signal is 17-18 Mbps, therefore, ATV signals can be recorded in this SD mode transparently.
FIG. 47A and FIG. 47B respectively show the head traces of a rotary head during normal playback and fast playback of a consumer digital VTR. In the figure, tracks are recorded slantwise by heads having different azimuth angles. During normal playback, the tape transport speed is the same as in recording, so the head traces the recorded tracks as shown in FIG. 47A. During fast playback however, the tape speed is different so the head cuts across several tracks and can only reproduce fragments of tracks having the same azimuth. FIG. 47B shows the case for 5 times speed playback (5.times. speed playback).
In an MPEG bit stream, only intra-encoded blocks are decoded independently, without referring to other frames. Assuming the MPEG2 bit stream is sequentially recorded on each track, intra-encoded data will be separated from intermittently reproduced data during fast playback, and the image will be reconstructed only from this separated intra-encoded data. The reproduced area on the screen will be discontinuous, and block fragments will be scattered over the screen. Further, as the bit stream is variable length encoded, there is no guarantee that the whole screen will be periodically updated, and some parts may not be updated for long periods. As a result, the image quality during fast playback is unsatisfactory, and is unsuitable for consumer digital VTR.
FIG. 48 is a block diagram of a conventional bit stream recording device that is capable of fast playback. Here, the video area of each track is divided into a main area for recording the bit stream of the whole ATV signal, and a duplication area for recording important parts of the bit stream (HP data) used in reconstructing the image during fast playback. During fast playback, only intra-encoded blocks are effective so these are recorded in the duplication area. However to further reduce the amount of data, low frequency components are extracted from all intra-encoded blocks and recorded as HP data. In FIG. 48, an input terminal 1 is for receiving the bit stream. An output terminal 2 is for outputting the bit stream. An output terminal 3 is for outputting the HP data.
The MPEG2 bit stream is input via the input terminal 1, output via the output terminal 2 and sequentially recorded in the main area. The bit stream from the input terminal 1 is also input to a variable length encoder 210, where the syntax of the MPEG2 bit stream is analyzed, and intra-images are detected, timing is generated by a counter 211. Low frequency components are extracted from all blocks in the intra-images by a data extractor 212, and EOB's are appended by an EOB (End of Block) appending circuit 213 so as to construct HP data which is recorded in the duplication area.
FIG. 49A and FIG. 49B are schematic diagrams showing the basic functions in the playback. During normal playback, all of the bit stream recorded in the main area 220 is reproduced and output to the MPEG2 decoder outside the digital VTR, while the HP data recorded in the duplication area 221 is discarded, as shown in FIG. 49A. On the other hand, during fast playback, only the HP data in the duplication area 221 is collected and sent to the decoder, while the bit stream in the main area 220 is discarded, as shown in FIG. 49B.
FIG. 50A and FIG. 50B show an example of head trace during fast playback. When the data speed is an integer multiple and phase-locked state is maintained, head scanning is synchronized with tracks having the same azimuth and the positions from which data is reproduced are fixed. In the figure, assuming that useful data is obtained from a part of the playback signal having an output level higher than -6 dB, the hatched areas will be reproduced by one head. FIG. 50A and FIG. 50B show the case of 9 times speed (9.times.speed), so signal reading of this hatched area is guaranteed at 9.times.speed. The HP data may therefore be recorded in this area. At other speeds however, signal reading is not guaranteed. The areas where the HP data is recorded must thus be chosen so that it can be read at any of several tape speeds.
FIG. 51 shows examples of scanning areas for three tape speeds at which the head is synchronized with tracks having the same azimuth, and overlapping areas (at the bottom of FIG. 51) at the three fast playback speeds (the areas which are reproduced at any of the three fast playback speeds). The locations where a duplication area is formed is selected from these overlapping areas, in such a manner that reading of HP data at different tape speeds is guaranteed. FIG. 51 shows the case of 4.times., 9.times. and 17.times.speed, forward playback, but these scanning areas are the same as those for -2.times., -7.times. and -15.times. speed playback (i.e., 2, 7 and 15 times reverse speed).
However, it is impossible for the head to trace exactly the same area at several tape speeds due to the fact that the number of tracks the head cuts across depends on the tape speed. Also, the head must be able to trace from any identical-azimuth track. FIG. 52 shows head traces at different tape speeds. FIG. 52 shows 5.times.and 9.times.speed head scanning trace in a conventional VTR. In the figure, areas 1, 2, 3 are selected from 5.times.and 9.times.speed overlapping areas. By repeatedly recording the same HP data on 9 tracks, the HP data can be read at either 5 or 9 times speed.
FIG. 53A and FIG. 53B show example of head traces at five 5.times.speed playback. As seen from FIG. 53A and FIG. 53B, by repeatedly recording identical HP data over the number of tracks equal to the speed multiplier (ratio of the fast playback speed to normal playback speed), the HP data can be reproduced by means of a head in synchronism with the track of the same azimuth. By repeating the duplication of HP data over the number of tracks equal to the speed multiplier corresponding to the maximum playback speed (the maximum of the fast playback speeds), the HP data can be reproduced at any playback speed, either in forward or reverse direction.
FIG. 54 shows examples of main and duplication areas. In a consumer digital VTR, the video area of each track comprises 135 sync blocks, the main area comprises 97 sync blocks and the duplication area comprises 32 sync blocks. For the duplication areas, overlapping areas corresponding to the 4.times., 9.times., and 17.times.speed playback shown in FIG. 51 are selected. In this case, the data rate for the main area is approx. 17.46 Mbps and as the same data is recorded 17 times in the duplication area, the data rate for the duplication areas is approx. 338.8 kbps.
As the conventional VTR has the construction described above, the data recorded in the duplication area 221 is always provided with an EOB. If the EOB code consists of four bits, four times the number of blocks per picture, i.e.,
4.times.46,080=184,320 bits are required. PA1 means for extracting header information from the bit stream; PA1 means for modifying the header information; PA1 means for detecting and extracting intra-coded block components from the bit stream; and PA1 means for configuring one track with an area for recording the input bit stream and an area for recording fast playback HP data; PA1 said extracting means extracting, as said intra-coded block components, a predefined number of orthogonal transform coefficients; and PA1 said configuring means configuring the fast playback HP data without appending EOB codes to the said intra-coded block components. PA1 means for extracting header information from the bit stream; PA1 means for modifying the header information; PA1 means for detecting and extracting intra-coded block components from the bit stream; and PA1 means for configuring one track with an area for recording the input bit stream and an area for recording fast playback HP data, PA1 said extracting means extracting, as said intra-coded block components, a predefined number of variable-length codes; and PA1 said configuring means configuring the fast playback HP data without appending EOB codes to said intra-coded block components. PA1 means for extracting header information from the bit stream; PA1 means for modifying the header information; PA1 means for detecting and extracting intra-coded block components from the bit stream; and PA1 means for configuring one track with an area for recording the input bit stream and an area for recording fast playback HP data; PA1 said extracting means extracting, as said intra-coded block components, a predefined number of orthogonal transform coefficients, and PA1 said configuring means selecting the shortest EOB code from said plurality of code maps, and configuring the fast playback HP data by appending the selected EOB code to said intra-coded components. PA1 means for extracting header information from the bit stream; PA1 means for modifying the header information; PA1 means for detecting and extracting intra-coded block components from the bit stream; and PA1 means for configuring one track with an area for recording the input bit stream and an area for recording fast playback HP data; PA1 said extracting means extracting, as said intra-coded block components, a predefined number of variable-length codes; and PA1 said configuring means selecting the shortest EOB code from said plurality of code maps, and configuring the fast playback HP data by appending the selected EOB code to said components. PA1 means for extracting fast playback signals from normal recording signals; PA1 means for recording the fast playback signals in predefined regions on predefined tracks; PA1 means for recording identification signals for identifying the tracks on which the fast playback signals are recorded; and PA1 means for reproducing the identification signals; PA1 said fast playback signal recording means recording the fast playback signal for each of the playback speeds on tracks predefined for each of the playback speeds.
The number of bits per picture is given by 1920 pels.times.1024 lines for 4:2:0 luminance/color difference components signals.
The EOB code indicates the end of the data of each DCT block, and is not directly related to the quality of the picture. It is undesirable to use 184,320 bits of the duplication areas 221 for the EOB codes which do not directly contribute to the quality of the picture.
In addition, when the track is non-linear or the scanning trace is non-linear during fast playback, or when there is an error in the recording start point on the track at the lower edge of the tape, the data in the fast playback region corresponding to these parts are not reproduced.
Moreover, since the data in a plurality of fast playback regions needs to be reproduced during one scan of the head, playback cannot be conducted at speeds which do not satisfy the above requirement. There is thus a limitation as to the speeds at which the fast playback is achieved.
Moreover, the rotation speed of the drum of a four-head configuration is half the the rotation speed of the drum of a two-head configuration, so that the angle between the scanning trace and the track is larger with the four-head drum, and the data in the fast playback regions which is reproduced with the two-head drum at a certain speed can be reproduced with the four-head drum at a speed half that of said certain speed.
Furthermore, since the fast playback data for all the playback speeds are used in common, and since the interval at which one frame of image data is reproduced and displayed at each fast playback speed depends on the time taken by the head to pass the region in the longitudinal direction of the tape in which one frame of image data is recorded, and the above-mentioned interval is therefore inversely proportional to the tape transport speed. It is thus difficult to display images which is easy to see at all the playback speeds.